1. Field of the Invention
The present invention relates to a data communication system including a plurality of low-speed transmission lines which are connected between high-speed transmission lines, and in particular to a transmission line switching system and an inverse multiplexing transmission method which are used for the system.
2. Description of the Related Art
As information processing and data transmission technologies progress, a demand for a higher transmission capability is growing while it is also demanded that a communication cost can be made as low as possible. In view of this cost/speed tradeoff, an inverse multiplexing technique is one of attractive communication schemes. Especially, an IMA (Inverse Multiplexing for ATM) technique has received widespread attention.
According to the IMA technique, in data transmission on a physical layer, an ATM cell stream on a high-speed transmission line is divided by an inverse multiplexer into a plurality of cell streams each travelling through a lower speed transmission line. The respective divided cell streams are sent through the plurality of lower speed transmission lines to a multiplexer. The divided cell streams are multiplexed by the multiplexer into the original ATM cell stream which is sent to the other high-speed transmission line. Such an IMA technique has been prepared by ATM Forum (see AF-PHY-0086.000).
In Japanese Unexamined Patent Publication No. 10-500271, there has been proposed an ATM inverse multiplexing technique. An outline of this conventional technique will be explained with reference to FIG. 1.
As shown in FIG. 1, an input stream of ATM cells (here, P0-P8) on a high-speed line is distributed among a plurality of low-speed lines (here, three low-speed lines: LS-Line1, LS-Line2, and LS-Line3) in a round robin fashion. Therefore, it is necessary for both of an ATM cell sending device and an ATM cell receiving device to operate according to the round robin order to successfully combine the distributed ATM cell streams on the low-speed lines into the original ATM cell stream. For this, the ATM cell sending device and the ATM cell receiving device transmit an OAM (operation administration and maintenance) cell including a cell sequence number and a field-back line status field.
In FIG. 1, for example, the ATM cell sending device transmits through the first low-speed line LS-Line1 a first OAM cell OAM1 followed by ATM cells P0, P3 and P6 in the order shown and thereafter a second OAM cell OAM2. It is the same with the other low-speed lines LS-Line2 and LS-Line3. In other words, the ATM cell sending device regularly inserts an OAM cell into each of the low-speed lines LS-Line1 through LS-Line3. On the other hand, the ATM cell receiving device sends back the OAM cell regularly. The cell sequence number included in the OAM cell is used to specify the round robin order and the field-back line status field thereof is used to indicate receive-ready status according to the specified round robin order. Furthermore, even if the number of low-speed cell lines which can be used in changed due to some failure, the round robin order can be regularly recognized using the OAM cell transmitted between the ATM cell sending and receiving devices.
According to the conventional transmission line switching system as mentioned above, the ATM cell receiving device reproduces the original ATM cell stream from the divided cell streams using the OAM cell which is regularly inserted. Therefore, it is necessary for the ATM cell receiving device to store the divided cells between the first and second OAM cells received through the low-speed lines.
However, in the case where arrival time deviations occur among the low-speed cell lines due to protocol control which is carried out for each of the low-speed cell lines as shown in FIG. 1, all the cells included in the range from the first cell P0 on the fastest line LS-Line1 to the last cell P7 on the slowest line LS-Line2 are stored in the ATM cell receiving device. More specifically, in FIG. 1, the cells to be stored consist of the distributed cells P0-P8 as well as five protocol waiting cells indicated by reference symbol P. Accordingly, the ATM cell receiving device needs a cell storing memory device having a large capacity.
Further, since the OAM cell is used as a unit of cell management, a failure/restoration detection interval at the receiving side highly depends on the position and the transmission interval of an OAM cell. Furthermore, in the case of an inverse-multiplexing device requiring the change of the setting of the intermediate transmission lines and the failure restoration mechanism, the operation using the OAM cell needs a complicated and time-consuming protocol. As a consequence, a delay in the detection of occurrence of some failure itself becomes large with an increase in number of the intermediate transmission lines and further, when a failure occurs on a line, it becomes difficult to switch over the line to another line as early as possible.
An object of the present invention is to provide a data transmission system and method which is capable of performing an inverse multiplexing operation with reduced amount of memory for storing data which are received through a plurality of intermediate transmission lines.
Another object of the invention is to provide a transmission system and a switching device which are capable of detecting the occurrence of a failure on the intermediate transmission lines in a shortest time possible and rapidly performing a switch-over of the intermediate transmission lines.
According to the present invention, in a system for transmitting a stream of data from a first transmission line to a second transmission line through a plurality of intermediate transmission lines, a line speed of each of the intermediate transmission lines is lower than that of each of the first and second transmission lines and a total line capacity of the intermediate transmission lines is larger than a line capacity of each of the first and second transmission lines. A method comprises the steps of:
a) dividing the stream of data received from the first transmission line into a plurality of data blocks;
b) distributing the plurality of data blocks together with next line identification information corresponding to each of the data blocks among the plurality of intermediate transmission lines, the next line identification information identifying an intermediate transmission line to which a next data block following the data block is to be transmitted;
c) receiving data blocks having the next line identification information added thereto from the plurality of intermediate transmission lines; and
d) reproducing the stream of data from the data blocks received from the plurality of intermediate transmission lines by referring to the next line identification information added to each of the data blocks to transmit the stream of data to the second transmission line.
According to another aspect of the present invention, a sending device connecting the first transmission line to the plurality of intermediate transmission lines divides the stream of data received from the first transmission line into a plurality of data blocks. Then the sending device distributes the plurality of data blocks together with next line identification information corresponding to each of the data blocks among the plurality of intermediate transmission lines, the next line identification information identifying an intermediate transmission line to which a next data block following the data block is to be transmitted. A receiving device connects the plurality of intermediate transmission lines to the second transmission line. The receiving device receives data blocks having the next line identification information added thereto from the plurality of intermediate transmission lines. Each of received data blocks is checked to see whether a failure occurs on any of the intermediate transmission lines. When a failure occurs on an intermediate transmission line, instructing the sending device to use the intermediate transmission lines other than the intermediate transmission line on which the failure occurs to distribute the plurality of data blocks.
According to further another aspect of the present invention, a method comprises the steps of:
at the sending device,
a) dividing the stream of data received from the first transmission line into a plurality of data blocks; and
b) distributing the plurality of data blocks together with next line identification information corresponding to each of the data blocks among the plurality of intermediate transmission lines, the next line identification information identifying an intermediate transmission line to which a next data block following the data block is to be transmitted.
at the receiving device,
c) receiving data blocks having the next line identification information added thereto from the plurality of intermediate transmission lines;
d) storing a past selection sequence of the intermediate transmission lines;
e) counting the number of times the next line identification information added to a received data block deviates from the past selection sequence; and
f) when a counter exceeds a predetermined value, updating the past selection sequence.